Pyruvic acid\xa0is first altered in the transition reaction by removal of a\xa0carbon\xa0and two oxygens (which form\xa0carbon dioxide). When the\xa0carbon dioxide\xa0is removed, energy is given off, and NAD+ is converted into the higher energy form NADH. Coenzyme A attaches to the remaining 2-C (acetyl) unit, forming acetyl Co-A.\xa0Pyruvic acid\xa0supplies energy to living cells through the citric\xa0acid\xa0cycle (also known as the Krebs cycle)\xa0when oxygen\xa0is present (aerobic respiration);\xa0when oxygen\xa0is lacking,\xa0it\xa0ferments to produce lactic\xa0acid.\xa0Pyruvate\xa0is an important chemical compound in biochemistry.